In this laboratory, triphenylmethanol was synthesised from reacting benzophenone and bromobenzene using Grignard reaction. As the reaction was to set up to produce a Grignard reagent and then recrystallize it to obtain pure sample. The percentage yield obtained was 55% and its melting point was 161 co which is within the literature value 160-163 co. In addition to that the IR spectroscopy confirmed the molecule structure to be triphenylmethanol. Introduction: The Grignard reagents are alkyl magnesium halides, R-Mg-X were first introduced in 1900.
These reagents are usually repared by reacting one atom with one molecule or by other means magnesium and alkyl halide in the presence of dry alcohol free ether. RX + Mg I RM9X In Grignard reactions ethereal solution was widely used in the first early stages and other solvents such as tertiary amines, dimethyl ether and tetrahydrofuran (THF). Since it was discovered that the THF increases the reactivity of organic halides towards magnesium. Grignard reagents were prepared by aryl chlorides and vinyl halides in the THF solvent. However, these days the best way to react it is to add diethylene dibromide and to use magnesium in excess and its known as he best entertainment technique.
Grignard reagents are widely used in the synthesis of organic molecules and organometallic compounds. Grignard reagents are extremely reactive with Lewis acids such as water. Therefore, precautions were taken during the lab to ensure that the reaction was not ruined due to the presence of water, increase the percentage yield, and obtain more accurate results. These reactions tend to split into two groups: Addition of the Grignard reagent to a compound containing multiple bond group such as: Which is the process of addition of the R group of RMGX occurs t the less electronegative and the MgX group at the higher electronegative atom.
Which means that the R group is the negative and the MgX group is the positive ends of the dipole in the RMGX. Double decomposition with compounds containing an active hydrogen atom or reactive hydrogen atom as an example below: When a compound consisting of an active hydrogen atom combined to an oxygen, nitrogen or sulphur react with a Grignard reagent, the alkyl group is converted into alkane. In addition to a very good example; forming an ester is possible by reacting Grignard reagent with alpha-monochloroether.
Grignard reagents could be used in forming many different compounds such as acid anhydrides, acids, alkyl cyanides, primary amines, alkyl halides, Thioalcohols, sulphuric acids and Dithioic acids. The most common synthetic uses are: Hydrocarbons: when a Grignard reagent is reacted with any compound containing active hydrogen, a hydrocarbon is formed in the presence of water or dilute acid as shown below Primary alcohols: a Grignard reagent may be used to form an alcohol by reacting it with dry oxygen and decomposing the product with acid Secondary alcohols: When a Grignard reagent is reacted with any aldehyde rather han a formaldehyde, a secondary alcohol is formed.
Tertiary alcohols: A tertiary alcohol with different alkyl groups may be prepared due to the Grignard reagent reacting with ketone as was made in this laboratory. Although the starting material could be R1COR2 and R2MgX, as tertiary alcohols are readily dehydrated to alkenes by acids, the complex is often broken up by aqueous ammonium chloride or buffered acid solution. Aldehydes: An aldehyde could be prepared by reacting Grignard reagent with ester instead of ethyl formate to avoid the formation of a secondary alcohol
Ketones: they are prepared due to a reaction between Grignard and a formic ester (tertiary alcohol) or by adding alkyl cyanide to Grignard reagent Reaction mechanism: Method: Firstly, added 1. 011g of magnesium turnings a dry 250 ml 2- neck flask, then grated a few of them against the side with a dry glass rod and assembled the apparatus. Turned on the water to feed the condenser. Consequently, 5 mL of dry diethyl ether and about 1 mL of a measured 4 mL of bromobenzene were added to the flask and loosely plugged the top of the condenser and the dropping funnel with the cotton wool. Started the reaction y warming in hot water. When the reaction has started the solution started to get cloudy, small bubbles were observed. Then added the rest of the bromobenzene, diluted with diethyl ether 25 mL, in small amounts at a time from the dropping funnel. Finally refluxed the mixture for 10 minutes, after which almost all the magnesium should have dissolved.
Removed the Grignard solution from the heat and cautiously, with swirling, dropwise adding a 7g solution of benzophenone drop wisely in 25 ml dry diethyl ether via the dropping funnel. Swirled for a further 5 minutes. Liberated the product from its magnesium alt by cautiously adding dilute 4 M hydrochloric acid until clear aqueous and ethereal layers remained and all solid has redissolved. Separated the layers then washed the aqueous layer with a little more diethyl ether and washed the combined ethereal layers with water. Then retain the aqueous waste until the product has formed. Last thing, dried the ethereal layers over anhydrous sodium sulfate, then filtered. Removed the ether on the rotary evaporator and recrystallised the residue from the minimum amount of methanol.
Results and calculation: Weight of weighing boat 0. 859g Weight of mg+ weighing boat 1. 70g Weight of mg used 1. 011g Weight of empty weighing boat 0. 623g Weight of boat + benzophenone 7. 685g Weight of benzophenone used 7. 062g Final product: Weight of empty sample bag 0. 939g Weight of bag + product 6. 377g Weight of product 5. 438g Calculation: Product Molar Mass Theoretical mass yield (g) Mass obtained(g) Number of moles Stoichiometry triphenylmethanol 260. 33 9. 882 5. 438 0. 021 1 %yield=actual mass/theoretical mass x100=(5. 438)/(9. 882) x100=55% Melting point=161-161. 4 °C Sample Transmittance (cm-1) Assignment (stretching frequencies) Typical Values (cm-1) triphenylmethanol 3490 059 1597,1489,1444 1008,756,730 O-H alcoholic sp3 C-H,aromatic CH2 aromatic C-H unsarturated out-of-plane bending 3600-2500 3100-2700 1050-670 Infrared spectrum: Discussion: The IR spectrum presented specific peaks at 3490 cm-1 which indicates the presence of alcohol, at area peak 3059 cm-1 carbon to hydrogen stretch is present. While at area peaks 1597, 1489 and 1444 cm-1 which represents a carbon to carbon stretch it also indicates the presence of aromatic bending.
Since, that Triphenylmethanol consists of an alcohol group and aromatic bending; according to the peaks in the IR spectrum, he product that was synthesized during the laboratory confirmed the functional groups that are present in triphenylmethanol. Umpohlung is a polarity inversion, by means it is a chemical modification of a functional group with an aim of the reversal of polarity of that group. As it has been applied in this experiment when the bromine was first the most electrophile in the molecule but by adding the magnesium to the PH-Br the polarity has changed or exchanged. As the magnesium became positively charged and the phenyl ring became the most electrophile instead of the bromine.
Conclusion: In this laboratory, 5. 438g of triphenylmethanol was obtained by reacting benzophenone and bromobenzene using Grignard reagent. The reaction obtained percentage yield of 55%w/w and melting point 161-161. 40C, which applies with the theoretical value range 160-1630C. in addition to the IR spectrum also confirmed the triphenylmethanol molecule structure at area peak 3490 cm-1 which represents the O-H group and at area peaks 1597,1489and1444 cm-1 shows the aromatic bonds. Therefore, the unkown product obtained was confirmed to be triphenylmethanol according to the IR spectrum and the BP limits.